瑜伽論記 9.316

9.316

言祖父者，如毘婆沙說：劫初時人其性淳直後漸澆薄，遂有一人先作盜業初生鬼道，由先生故故名祖父。次後生者，從初得名皆名祖父。

Regarding the term "grandfather" (pitāmaha): as explained in the *Vibhāṣā*, at the beginning of the kalpa, human nature was honest and straightforward, but later gradually became frivolous and shallow. There was then one person who first committed the karma of theft and was the first to be born into the realm of pretas. Because he was born first, he was named "grandfather." Those born subsequently all received the name "grandfather" following this initial one.